1. Field of the Invention
The present invention is related to an infant ventilator for artificial ventilation used in different situations in neonatal and pediatric wards, as well as to exhalation valves used in such a device. The invention can be used whenever there is a need to maintain a constant respiratory activity in a patient by means of mechanical ventilation.
2. Brief Description of the Prior Art
Modern ventilators used for pediatric and neonatal care are of continuous flow, with limited time cycles and pressure. In this type of ventilator, a preset mixture of air and oxygen flows continuously and constantly through a tube known as an inspiratory tube until it reaches the patient. This mixture of air passes through a moisturizing system, and from this system, through another tube called an expiratory tube, to an exhalation valve at the end of the expiratory airway. During the inspiratory phase, the valve is opened during expiration and closed during inspiration. This allows a continuous flow of the mixture to pass through the tubes to the patient, or, during the expiratory phase, to the environment when the air accumulated in the lungs during the inspiratory phase is exhaled together with the continuous flow.
While closed, the exhalation valve maintains a closing pressure or an inspiratory pressure which may be adjusted, in order to maintain a constant limited pressure in the patient's system during the inspiratory phase. Thus, while open, it is possible to maintain an expiratory pressure, also adjustable, defining a residual pressure during expiration within desired preset limits.
The duration of inspiratory and expiratory phases is obtained through electronic circuits and/or pneumatic circuits which open and close the exhalation valve. The exhalation valve at the end of the expiratory airway is the active element responsible for the morphologies, pressure levels, and switching of the inspiratory and expiratory phases.
In addition to the essential devices necessary to operate the ventilator, according to the technique hereinabove described, prior art ventilators include systems for monitoring and/or controlling pressure, inspiratory and expiratory times, the ratio between inspiratory and expiratory times, etc. Each model is equipped with an alarm system. These alarm systems range from basic to complex. The basic alarm system is capable of detecting the drop in an inspiratory pressure where the complex alarm system is capable of measuring the minimum and the maximum inspiratory pressure and the time of apnea.
The ventilators currently available have some inherent problems. During the expiratory phase, the patient exhales air accumulated in his lungs during the inspiratory phase, and this air is exhausted together with the air flowing continuously through the system. During expiration, the resistances of the expiratory tube and of the exhalation valve itself result in a residual pressure which is relatively high due to the increased flows employed, to the point of not being able to reduce to atmospheric pressure even during a long expiration. The common solution for this problem is to install a venturi system at the end of the expiratory airway.
To reduce residual pressure, the state of the art ventilators employ an offsetting subatmospheric pressure, promoting a more efficient exhalation through the expiratory airway. In spite of obtaining a baseline with no residual pressure, the use of venturi is known to cause fluctuations in the pressure curves when switching phases. These fluctuations introduce a characteristic noise in the system during the valve's functioning.
As noted, the state of the art ventilators have an exhalation valve placed at the end of the expiratory airway, which effectively controls the pressures in the system. The pressure control is directly affected by the flow and resistance of the respiratory system components, as there already exists a pressure differential between the patient's connection and the end of the expiratory airway, due to the flow and resistance of the expiratory airway. In addition, higher flows generate higher turbulence which affect the valve control.
Another shortcoming of the use of a single valve at the end of the expiratory airway is the total loss of pressure control whenever there is an obstruction in the tubes of the expiratory airway, which will be reflected in an increase of the working pressure over the patient. A severe obstruction will increase the pressure above safety limits capable of rupturing the walls of the lungs, i.e., the occurrence of a pneumothorax. This is one of the most feared problems in neonatal care. In order to solve this problem, the state of the art ventilators use preset or adjustable relief valves which limit the highest pressure in the respiratory system. These valves need to be adjusted before the ventilator is used. However, according to medical statistics, this does not always happen.